Wavelength-variable laser

ABSTRACT

An optical semiconductor device outputting a predetermined wavelength of laser light includes a quantum well active layer positioned between a p-type cladding layer and an n-type cladding layer in thickness direction. The optical semiconductor device includes a separate confinement heterostructure layer positioned between the quantum well active layer and the n-type cladding layer. The optical semiconductor device further includes an electric-field-distribution-control layer positioned between the separate confinement heterostructure layer and the n-type cladding layer and configured by at least two semiconductor layers having band gap energy greater than band gap energy of a barrier layer constituting the quantum well active layer. The optical semiconductor device is applied to a ridge-stripe type laser.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.16/672,163, filed Nov. 1, 2019, which is a continuation-in-part of U.S.patent application Ser. No. 16/029,285, filed Jul. 6, 2018, now U.S.Pat. No. 10,511,150, which is a continuation-in-part of U.S. patentapplication Ser. No. 15/454,444, filed Mar. 9, 2017, now U.S. Pat. No.10,020,638, which is a continuation of U.S. patent application Ser. No.14/795,387, filed Jul. 9, 2015, now U.S. Pat. No. 9,601,905, which is acontinuation of U.S. patent application Ser. No. 14/507,374, filed Oct.6, 2014, now U.S. Pat. No. 9,083,150, which is a continuation ofInternational Application No. PCT/JP2013/060391 filed on Apr. 4, 2013,which claims the benefit of priority from U.S. Provisional PatentApplication No. 61/621,013 filed on Apr. 6, 2012, the entire contents ofall of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a wavelength-variable laser, asemiconductor laser module, and an optical fiber amplifier.

2. Description of the Related Art

In recent years, increase in data traffic due to rapid spread ofInternet or rapid increase in connections among intra-company LANs, etc.is a problem. Then, in order to solve that problem, a wavelengthdivision multiplexing (WDM) system has achieved a remarkable developmentand become popular. In the WDM system, by multiplexing a plurality ofsignals onto lights each having different wavelength, large capacitytransmission which is equal to or greater than 100 times relative toconventional cases is realized by using an optical fiber. In particular,in the WDM system, optical amplification by an optical fiber amplifiersuch as an erbium-doped optical fiber amplifier (hereinafter, EDFA) or aRaman amplifier etc. is indispensable, this optical amplificationenables wide-band and long-haul transmission. Herein an EDFA is anoptical fiber amplifier to which the principles applied that, wheninputting pumping light having a wavelength of 1480 nm or a wavelengthof 980 nm etc. from a pumping laser into a special optical fiber(hereinafter EDF) which is doped with erbium which is rare earth, 1550nm band wavelength of light which is simultaneously-inputtedtransmission signal light is amplified in the above-described EDF.

Also, as a form of using EDFA, a so-called remote-pump-type is proposed,in which, when amplifying signal light in the middle of a transmissionoptical fiber laid on seabed, a pumping laser is disposed on land andpumping light outputted from the pumping laser is made input into theEDF through the transmission optical fiber. In the remote-pump-typeEDFA, the pumping laser can be maintained or replaced easily bydisposing the pumping laser on land.

On the other hand, a Raman amplifier is an optical fiber amplifier of adistributed type which, unlike an EDFA, does not need a special opticalfiber such as an erbium-doped optical fiber but uses an optical fiber ofan ordinary transmission path as a gain medium. Since the Ramanamplifier has flat gain in wide-band, as compared with a WDMtransmission system based on a conventional EDFA, the Raman amplifierhas a feature capable of realizing wide-band transmission band. Itshould be noted that, in the Raman amplifier, since its amplificationgain is lower than that of the EDFA, its pumping laser is required tohave high output characteristics equal to or greater than that of theEDFA.

Therefore, in order to realize improvement in stability of the WDMsystem or reduction of the number of repeating, the pumping laser isrequired to have stable and high optical output capability. As thepumping laser, semiconductor laser devices having various structuressuch as a buried hetero (BH) structure etc. are used, and currently, forthe above-described reason, development for especially a high poweroutput semiconductor laser device is ongoing actively (see Jan P. vander Ziel, et. al., “InGaAsP(λ=1.3 μm) Stripe Buried HeterostructureLasers Grown by MOCVD,” IEEE JOURNAL OF QUANTUM ELECTRONICS VOL. 27, NO.11, pp. 2378-2385, 1991, Japanese Patent Application Laid-open No.2000-174394, and Japanese Patent No. 3525257, which are referred as aNon-Patent document 1, Patent document 1 and Patent document 2,respectively).

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

In accordance with one aspect of the present invention, awavelength-variable laser outputting a predetermined wavelength of laserlight includes: a quantum well active layer positioned between a p-typecladding layer and an n-type cladding layer in thickness direction; aseparate confinement heterostructure layer positioned between thequantum well active layer and the n-type cladding layer; and anelectric-field-distribution-control layer positioned between theseparate confinement heterostructure layer and the n-type cladding layerand configured by at least two semiconductor layers having band gapenergy greater than band gap energy of a barrier layer constituting thequantum well active layer.

In accordance with another aspect of the present invention, asemiconductor laser module includes: a wavelength-variable laser whichis a semiconductor laser element outputting a predetermined wavelengthof laser light which includes: a quantum well active layer positionedbetween a p-type cladding layer and an n-type cladding layer inthickness direction; a separate confinement heterostructure layerpositioned between the quantum well active layer and the n-type claddinglayer; and an electric-field-distribution-control layer positionedbetween the separate confinement heterostructure layer and the n-typecladding layer and configured by at least two semiconductor layershaving band gap energy greater than band gap energy of a barrier layerconstituting the quantum well active layer. The semiconductor lasermodule includes: a temperature-control module controlling temperature ofthe semiconductor laser element; an optical fiber guiding the laserlight outputted from the semiconductor laser element to outside; and anoptical-coupling lens system coupling the semiconductor laser elementand the optical fiber optically.

In accordance with still another aspect of the present invention, anoptical fiber amplifier includes: a semiconductor laser module whichincludes a wavelength-variable laser which is a semiconductor laserelement outputting a predetermined wavelength of laser light whichincludes: a quantum well active layer positioned between a p-typecladding layer and an n-type cladding layer in thickness direction; aseparate confinement heterostructure layer positioned between thequantum well active layer and the n-type cladding layer; and anelectric-field-distribution-control layer positioned between theseparate confinement heterostructure layer and the n-type cladding layerand configured by at least two semiconductor layers having band gapenergy greater than band gap energy of a barrier layer constituting thequantum well active layer. The semiconductor laser module includes: atemperature-control module controlling temperature of the semiconductorlaser element; an optical fiber guiding the laser light outputted fromthe semiconductor laser element to outside; and an optical-coupling lenssystem coupling the semiconductor laser element and the optical fiberoptically. The optical fiber amplifier includes: an amplificationoptical fiber including amplification medium; and an optical couplermultiplexing inputted signal light and the laser light outputted fromthe semiconductor laser module and making the multiplexed light inputinto the amplification optical fiber.

In accordance with still another aspect of the present invention, anoptical fiber amplifier includes: a semiconductor laser module whichincludes a wavelength-variable laser which is a semiconductor laserelement outputting a predetermined wavelength of laser light whichincludes: a quantum well active layer positioned between a p-typecladding layer and an n-type cladding layer in thickness direction; aseparate confinement heterostructure layer positioned between thequantum well active layer and the n-type cladding layer; and anelectric-field-distribution-control layer positioned between theseparate confinement heterostructure layer and the n-type cladding layerand configured by at least two semiconductor layers having band gapenergy greater than band gap energy of a barrier layer constituting thequantum well active layer. The semiconductor laser module includes: atemperature-control module controlling temperature of the semiconductorlaser element; an optical fiber guiding the laser light outputted fromthe semiconductor laser element to outside; and an optical-coupling lenssystem coupling the semiconductor laser element and the optical fiberoptically. The optical fiber amplifier includes: an optical fibertransmitting signal light; and an optical coupler making the laser lightoutputted from the semiconductor laser module input into the opticalfiber. In the optical fiber amplifier, an optical amplification isperformed by Raman amplification.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section, which is in parallel with an outputtingsurface, of a semiconductor laser element according to an embodiment 1;

FIG. 2 is a cross section, which is in the longitudinal direction of thesemiconductor laser element according to the embodiment 1.

FIG. 3 is a drawing showing a band diagram of the semiconductor laserelement according to the embodiment 1.

FIG. 4A is a drawing showing a stripe shape in the waveguide directionof light in the semiconductor laser element according to the embodiment1;

FIG. 4B is a drawing showing a stripe shape in the waveguide directionof light in a comparison element;

FIG. 5 is a drawing showing current-optical output characteristics ofthe semiconductor laser element according to the embodiment 1 and thecomparison element;

FIG. 6 is a drawing showing dependences of electric input power onoptical output of the semiconductor laser element according to theembodiment 1 and the comparison element;

FIG. 7 is a drawing showing a relationship between an active layer widthand a rate of occurrence of IL kink of each of the semiconductor laserelements according to embodiments 1, 2-1, and 2-2;

FIG. 8 is a drawing showing dependence of threshold current on theactive layer width of each of the semiconductor laser elements accordingto the embodiments 1, 2-1, and 2-2 and the comparison element;

FIG. 9 is a drawing showing dependence of optical output (when drivingat 1.8 A) on the active layer width of each of the semiconductor laserelements according to the embodiments 1, 2-1, and 2-2 and the comparisonelement;

FIG. 10 is a drawing showing dependence of driving voltage (when drivingat 1.8 A) on the active layer width of each of the semiconductor laserelements according to the embodiments 1, 2-1, and 2-2 and the comparisonelement;

FIG. 11 is a drawing showing dependence of differential resistance (whendriving at 1.8 A) on the active layer width of each of the semiconductorlaser elements according to the embodiments 1, 2-1, and 2-2 and thecomparison element;

FIG. 12 is a side cross section showing the structure of a semiconductorlaser module according to an embodiment 3;

FIG. 13 is a view showing a relationship between optical output at anend of a fiber and driving current of a semiconductor laser module usingthe element according to the embodiment 1 or the comparison element;

FIG. 14 is a view showing a relationship between optical output at theend of the fiber and power consumption of the semiconductor laser moduleusing the element according to the embodiment 1 or the comparisonelement.

FIG. 15 is a view showing a relationship between module powerconsumption and thermistor temperature of the semiconductor laser moduleusing the element according to the embodiment 1 or the comparisonelement at a time of 500 mW optical output from an end of the fiber;

FIG. 16 is a block diagram showing a configuration of an optical fiberamplifier according to an embodiment 4;

FIG. 17 is a block diagram showing a modification example 1 of theoptical fiber amplifier according to the embodiment 4;

FIG. 18 is a block diagram showing a configuration of a modificationexample 2 of the optical fiber amplifier according to the embodiment 4,which adopts forward pumping method;

FIG. 19 is a block diagram showing a modification example 3 of theoptical fiber amplifier according to the embodiment 4;

FIG. 20 is a block diagram showing a configuration of a modificationexample 4 of the optical fiber amplifier according to the embodiment 4,which adopts bi-directional pumping method;

FIG. 21 is a block diagram showing a modification example 5 of theoptical fiber amplifier according to the embodiment 4; and

FIG. 22 is a block diagram showing a schematic configuration of a WDMcommunication system using the optical fiber amplifier according to theembodiment 4 or a modification example thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of a wavelength-variable laser, a semiconductorlaser module, and an optical fiber amplifier according to the presentinvention will be explained in detail with reference to the drawings. Itshould be noted that the present invention is not limited by theseembodiments. Also, in each drawing, if deemed appropriate, identical orequivalent elements are given same reference numerals. In addition, itshould be noted that the drawings are schematic depictions, and may notrepresent the actual relation of dimension of each element.

Furthermore, different drawings may include portions using differentscales and dimensional relations.

The following problem occurs along with improvement of enhancing poweroutput of semiconductor laser device constituting a pumping laser. Thatis, along with increase in driving current and driving voltage alongwith improvement of enhancing power output of a pumping light source,increase in power consumption of a pumping light source occurs. Thisraises a concern that power consumption increases in not only in opticalcommunication systems but also in air-conditioning systems for coolingthe systems. Large-capacity and ultra-high-speed communication systemsare required to reduce power consumption.

In general, there are mainly the following methods of reducing powerconsumption of a semiconductor laser element.

1) a method of reducing electric resistance and thermal resistance byextending an active layer width while maintaining a single transversemode

2) a method of, by introducing an asymmetric electric field distributionstructure in which an optical electric field distribution is distributedat an n-type cladding layer side, reducing the electric fielddistributed at a p-type cladding layer side, intending higher efficiencyof external differential quantum efficiency of an element by reducinginternal loss along with reduction in inter valence band absorption, andreducing power consumption by reducing driving current and drivingvoltage.

Respective arts are disclosed by the Non-Patent document 1 regarding theabove described 1), the Patent document 1 regarding the above described2), and the Patent document 1 regarding a combination of the abovedescribed 1) and 2). However, these publicly known arts have thefollowing problems.

(Non-Patent Document 1)

It discloses that a waveguide layer is disposed under a GaInAsP activelayer, the center of an optical electric field distribution is shiftedto a substrate side, and it is operable with a single transverse modeeven at 5 μm width of the active layer. However, it is a problem thatthreshold current increases with the increasing thickness of thewaveguide layer and characteristic temperature of the threshold currentdecreases. This raises a problem that increase in threshold carrierdensity, increase in non-radiative component due to Auger effect at hightemperature, and leakage current to the waveguide layer cause thethreshold current to increase. Also, there is a problem that, sincequantum efficiency does not depend on the thickness of the waveguidelayer, loss in the waveguide layer limits the external differentialquantum efficiency.

(Patent Document 1)

As a method of solving a problem of optical loss due to inter valenceband absorption, an art of disposing an optical-field-controlling layer,of which refractive index is higher than that of its n-type claddinglayer and closer to refractive index of an active layer, inside then-type cladding layer, shifting the distribution of light to the n-typecladding layer side, and reducing the amount of light distributed in thep-type cladding layer.

However, the structure of the optical-field-controlling layer is thesame as that of the active layer.

As a result, there is a problem that, when being disposed at a positionfar from a separate confinement heterostructure (SCH) layer, anotheroptical waveguide is formed and the distribution of light becomesdual-peak.

Therefore, although this optical-field-controlling layer must bedisposed in the vicinity of a lower-portion SCH layer, if anoptical-field-controlling layer of which refractive index is high likethis is disposed in the vicinity of the lower-portion SCH layer, thereis a problem that, equivalent refractive index of the entire waveguidebecomes high, operation with a single transverse mode becomes difficult,and a high-order transverse mode occurs.

Also, this high-order transverse mode can be prevented by narrowing thewidth of a region including the active layer and the SCH layer. However,if the width of the region including the active layer and the SCH layeris narrowed in this manner, since electric resistance and thermalresistance of an element cannot be reduced, there is a problem thatoptical output saturation occurs due to heat generated at the activelayer when being highly injected.

(Patent Document 2)

It is an object to provide a semiconductor light-emitting element whichcan obtain high power output with a simple configuration and modetransfer hardly occurs even if optical confinement coefficient for anactive layer is lowered. A semiconductor light-emitting element for thispurpose is characterized in that a multi quantum well active layersandwiched between an n-type cladding layer and a p-type cladding layermade from InP is provided on an InP substrate, and that the n-typecladding layer is made from InGaAsP.

Therefore, it is configured that increase, in light loss due to intervalence band light absorption in the p-type cladding layer, caused bylowering optical confinement coefficients in the active layer and an SCHlayer can be restrained, and high power output laser light can beobtained.

Also, since refractive index difference between the active layer and then-type cladding layer becomes smaller than that of a conventional art,the maximum active layer width that can restrain high-order transversemode can be extended; and therefore, it is advantageous to enable thesemiconductor laser element to output higher power.

However, the n-type cladding layer is set at a thickness ofapproximately 7.5 μm. It is usually difficult to form that thick n-typecladding layer by matching lattice constant of InGaAsP with that of InPwhile restraining crystal defects. In particular, in case of 0.95 μm ofcomposition wavelength, there is a problem that ratios of Ga and Asbecome minute with respect to In or P; thus, difficulty in compositioncontrol increases further. In addition, there is a problem thatlaminating the n-type cladding layer takes approximately three hours;thus manufacturing time takes too long. Also, it is a problem that apart of sediment which adheres to an inner wall of the reactor for MOCVDthen adheres to a surface of the growth substrate as particle and causesepitaxial surface defects. As a result, the number ofqualitatively-acceptable elements obtained from a wafer reduces; thus,it is a problem for cost reduction.

In contrast to this, embodiments shown as follows are effective to atleast one of the above described problems.

A semiconductor laser element according to an embodiment 1 of thepresent invention will be explained. The semiconductor laser elementaccording to the present embodiment 1 is a high power-outputsemiconductor laser element having buried hetero structure and beingused as a pumping laser, and includes anelectric-field-distribution-control layer between an SCH layer and ann-InP cladding layer positioned under a quantum well active layer. Ascompared with a structure having no electric-field-distribution-controllayer, this enables higher power output operation by reducing opticalelectric field distribution to the p-InP cladding layer, reducing intervalence band absorption, and increasing external differential quantumefficiency.

The electric-field-distribution-control layer, configured by asemiconductor layer having band gap energy greater than band gap energyof a barrier layer constituting the quantum well active layer, restrainsabsorption of laser light emitted from the quantum well active layer andenables high power output operation.

It is preferable that the electric-field-distribution-control layer isconfigured with composition having band gap energy between band gapenergy of the n-InP cladding layer and band gap energy of the barrierlayer. This becomes a structure in which injected carrier is difficultto be trapped by the electric-field-distribution-control layer andlowering of efficiency in injecting carrier into the quantum well activelayer is restrained. As a result, increase in threshold current,lowering of efficiency, and increase in resistance due to introductionof the electric-field-distribution-control layer can be restrained.

Also, in case where the semiconductor laser element is of the BHstructure, by introducing the electric-field-distribution-control layerunder a current-blocking layer which constitutes the current-confiningstructure and making the current-confining structure and theelectric-field-distribution-control layer overlap in the in thicknessdirection, equivalent refractive index difference between the activelayer region and the current-blocking region (current-confining region)can be reduced.

As a result, it is possible to extend the active layer width whilemaintaining the single transverse mode. By doing this way, sinceelectric resistance and thermal resistance can be reduced, ahigh-power-outputting and low power-consuming laser device, in whichthermal saturation is restrained, can be realized.

In particular, by configuring the electric-field-distribution-controllayer with the InP layer as a first semiconductor layer which issemiconductor material having band gap energy identical with that of then-type cladding layer and a GaInAsP layer as a second semiconductorlayer made from semiconductor material of which band gap energy isgreater than that of the barrier layer constituting the quantum wellactive layer, the above described equivalent refractive index differencecan be reduced than in a case of configuring theelectric-field-distribution-control layer only with the GaInAsP layer.By doing this, a single transverse mode laser device having wider activewidth can be realized, and a high power-outputting and low powerconsuming laser device in which thermal saturation hardly occurs can berealized.

Also, by reducing the sum of the layer thicknesses of the GaInAsP layersas the second semiconductor layer constituting theelectric-field-distribution-control layer to be lower than 1 μm, anelectric-field-distribution-control layer having fewer crystal defectscan be realized. Also, in order to obtain sufficient effect as theelectric-field-distribution-control layer, it is preferable to set thesum of the layer thicknesses of the GaInAsP layers to be equal to orgreater than 100 nm. Also, by setting band gap composition wavelength ofthe GaInAsP layer as the second semiconductor layer constituting theelectric-field-distribution-control layer to be equal to or greater than1 μm, since it is possible to control Ga composition and As compositioneasily, lowering of manufacturing yield can be restrained. Furthermore,since it is possible to set the thickness of theelectric-field-distribution-control layer at approximately 3 μm, it canbe grown in a shorter period, i.e., approximately 40% of period forgrowing 7.5 μm-thickness of an n-GaInAsP-cladding layer as disclosed byPatent Document 2. By doing this way, it is possible to realize a laserdevice which is suitable for mass-production at lower cost.

FIG. 1 is a cross section, which is in parallel with a output surface,of a semiconductor laser element according to an embodiment 1. FIG. 2 isa cross section in the longitudinal direction cut along a line A-A′ ofFIG. 1 . The semiconductor laser element shown in FIGS. 1 and 2 is aFabry-Perot type laser to which a so-called buried hetero structure isapplied as a current-confining structure.

That is, the n-InP cladding layer 2, anelectric-field-distribution-control layer 12, and a gradedindex-separate confinement heterostructure multi quantum well(GRIN-SCH-MQW) active layer 3 are layered in this order on an n-InPsubstrate 1.

The electric-field-distribution-control layer 12 in an upper portion ofthe n-InP cladding layer 2 and the GRIN-SCH-MQW active layer 3 form amesa-stripe-like structure of which longitudinal direction is along alight-outputting direction. A p-InP current block layer 8 and an n-InPcurrent block layer 9 as the current-confining structure are layered inthis order adjacent to two sides with respect to a width direction ofthis mesa-stripe structure. A p-InP cladding layer 6 and a p-GaInAsPcontact layer 7 are layered on the n-InP current block layer 9. Also, ap-side electrode 10 is formed on the p-GaInAsP contact layer 7 and ann-side electrode 11 is formed on a back surface of the n-InP substrate1. Furthermore, as shown in FIG. 2 , a output-side reflecting coating 15is formed on a laser-light-output facet from which laser light isoutputted, and a reflection-side reflecting coating 14 is formed on areflecting facet facing the radiation-side reflecting coating 15.

Function of each layer described above will be explained briefly asfollows. Firstly, since the n-InP cladding layer 2 has refractive indexlower than effective refractive index of the GRIN-SCH-MQW active layer3, the n-InP cladding layer 2 has a function of confining lightgenerated from the GRIN-SCH-MQW active layer 3 in a vertical (thickness)direction.

The p-InP current block layer 8 and the n-InP current block layer 9carry out a function of confining current injected from the p-sideelectrode 10, confining light generated from the GRIN-SCH-MQW activelayer 3 in a transverse (width) direction, and realizing a singletransverse mode operation in which a high order of lateral transversemode is restrained. Since the p-side electrode 10 serves as an anode inthe semiconductor laser element according to the present embodiment 1,in case where voltage is applied between the p-side electrode 10 and then-side electrode 11, reverse-bias is applied between the n-InP currentblock layer 9 and the p-InP current block layer 8. Therefore, currentdoes not flow from the n-InP current block layer 9 toward the p-InPcurrent block layer 8, and current injected from the p-side electrode 10is confined and flows into the GRIN-SCH-MQW active layer 3.

The reflection-side reflecting coating 14 and the output-side reflectingcoating 15 are mirrors for forming a cavity, and their opticalreflectivity are optimized according to cavity length. Herein it shouldbe noted that reflectivity of the reflection-side reflecting coating 14is set at 95% and reflectivity of the output-side reflecting coating 15is set at 1.5%. It should be noted that, although reflectivity of thereflection-side reflecting coating 14 is set at 95% for laser light, thelaser light outputted from this facet is used as monitor light forstabilizing output when operating the semiconductor laser module usingthis semiconductor laser element. Therefore, although reflectivity ofthe reflection-side reflecting coating 14 is determined according to asystem to which the semiconductor laser element is applied,characteristics as the high power output laser device is not influencedif the reflectivity is equal to or greater than 90%.

In addition, in order to obtain higher power output, it is desirablethat cavity length L is equal to or greater than 800 μm, and forexample, it may be 1000 μm or 1300 μm.

The cavity length of the semiconductor laser element according to thepresent embodiment 1 is set at L=2000 μm.

The structure formed by each above-described layer is similar to aconventional buried hetero structure and can be replaced by knownmaterial, which is other than the above-described material or conductivetype, formed with the similar structure. The present embodiment 1 ischaracterized in that the above-describedelectric-field-distribution-control layer 12 is provided in an upperportion of the n-InP cladding layer 2 in such a buried hetero structure.

Hereinafter, function and effect of theelectric-field-distribution-control layer 12 will be explained.

The electric-field-distribution-control layer 12 is disposed between alower side SCH layer constituting the GRIN-SCH-MQW active layer 3 andthe n-InP cladding layer 2 and is formed with material having band gapenergy greater than energy of light according to oscillation wavelength.

As compared with a structure having noelectric-field-distribution-control layer 12, theelectric-field-distribution-control layer 12 reduces optical electricfield distribution in the p-InP cladding layer 6, reduces inter valenceband absorption, increases external differential quantum efficiency, andenables high power output operation.

Herein, by configuring the electric-field-distribution-control layer 12with a semiconductor layer having band gap energy which is greater thanband gap energy of the barrier layer constituting the GRIN-SCH-MQWactive layer 3, it is possible to restrain absorption of laser lightemitted from the active layer and enable high power output operation.

Furthermore, the electric-field-distribution-control layer 12 hascomposition of which band gap energy is between those of the n-InPcladding layer and the barrier layer. By doing this way, a structure isobtained in which injected carrier is hardly trapped in theelectric-field-distribution-control layer 12 and reduction in efficiencyfor injecting carrier into the GRIN-SCH-MQW active layer 3 isrestrained. By doing this way, increase in threshold current, reductionin efficiency, and increase in resistance due to introduction of theelectric-field-distribution-control layer 12 can be restrained.

Also, since equivalent refractive index difference between the activelayer region and the current-blocking region can be reduced byintroducing the electric-field-distribution-control layer 12 in a lowerlayer of the p-InP current block layer 8 and the n-InP current blocklayer 9 and making the p-InP current block layer 8 and the n-InP currentblock layer 9 overlap with the electric-field-distribution-control layer12 in the thickness direction, it is possible to extend the active layerwidth while maintaining the single transverse mode. By doing this way,since it is possible to reduce electric resistance and thermalresistance, a low-power-consuming and high-power-outputting laser deviceof which thermal saturation is restrained can be realized.

In particular, by forming the electric-field-distribution-control layer12 with the InP layer as the first semiconductor layer which issemiconductor material having band gap energy identical with that of then-InP cladding layer 2 and the GaInAsP layer as the second semiconductorlayer which is made of semiconductor material of which band gap energyis greater than that of the barrier layer constituting the GRIN-SCH-MQWactive layer 3, the above described equivalent refractive indexdifference can be lower than a case where theelectric-field-distribution-control layer 12 is formed only with theGaInAsP layer. By doing this way, a single transverse mode laser devicehaving wider active width can be realized and a low-power-consuming andhigh-power-outputting laser device in which thermal saturation hardlyoccurs can be realized.

Also, by reducing the sum of the layer thicknesses of the GaInAsP layersas the second semiconductor layer constituting theelectric-field-distribution-control layer 12 to 1 μm, theelectric-field-distribution-control layer 12 having less crystal defectscan be realized.

In particular, since it is possible to control Ga composition and Ascomposition easily by setting the band gap composition wavelength of theGaInAsP layer as the second semiconductor layer constituting theelectric-field-distribution-control layer 12 at equal to or greater than1 μm, it is possible to restrain lowering of the manufacturing yield.Furthermore, since it is possible to set the thickness of theelectric-field-distribution-control layer 12 at approximately 3 μm, andtherefore, since it can be grown in a shorter period, i.e.,approximately 40% of period for growing 7.5 μm-thickness of ann-GaInAsP-cladding layer as disclosed by Patent Document 2, a low-costlaser device which is suitable for mass-production can be realized.

An example of a process manufacturing a semiconductor laser elementaccording to the embodiment 1 shown in FIG. 1 will be explained asfollows.

To start with, the n-InP cladding layer 2 which is made of InP having0.5 μm of thickness and 1 to 2×10¹⁸/cm³ of density of impurity is formedon the n-type InP semiconductor substrate 1 by using metal-organicchemical vapor deposition (MOCVD) method.

After that, the electric-field-distribution-control layer 12 configuredto have 2.6 μm of thickness is formed.

The electric-field-distribution-control layer 12 in this state isconfigured with 4 cycles of a GaInAsP layer having 0.95 μm of composingwavelength and 20 nm of thickness, an InP layer having 180 nm ofthickness, a GaInAsP layer having 1.0 μm of composing wavelength and 40nm of thickness, and an InP layer having 160 nm of thickness, and with 4cycles of a GaInAsP layer having 1.1 μm of composing wavelength and 16nm of thickness and an InP layer having 384 nm of thickness. That is, itis a structure in which the total thickness of the GaInAsP layerincluded in the electric-field-distribution-control layer 12 is 244 nmand accounts for 9.4% of the total thickness (2600 nm) of theelectric-field-distribution-control layer 12. Also, in the presentembodiment 1, the electric-field-distribution-control layer 12 is dopedwith 1×10¹⁸/cm³ of density of impurity to be an n-typeelectric-field-distribution-control layer.

Herein it is desirable that the sum of the layer thicknesses of then-InP cladding layers are approximately 0.5 μm to 3.5 μm. A lower limitvalue is determined from a view point for a buffer layer for restrainingdislocation from a substrate, and an upper limit value is determinedfrom a view point for load to a crystal-growing equipment caused bylonger crystal-growing time and manufacturing cost.

In case of configuring the first semiconductor layer with InP as amaterial, it is preferable that each layer thickness of the firstsemiconductor layer is 10 to 50 nm.

When the layer thickness is thinner than 10 nm, quantum size effectbecomes noticeable. In addition, mutual diffusion of group V atomsoccurs at an interface of the first semiconductor layer and the secondsemiconductor layer by heat applied in a regrowth process for forming aBH structure or in thermal treatment process at a process of formingelectrodes. Although this forms a quantum status in which an electrontransits at a wavelength shorter than designed, when the layer thicknessis thinner, effect by shift to shorter wavelength by the quantum sizeeffect becomes noticeable. Also, in some cases, effect by theelectric-field-distribution-control layer may vary due to variation ofgrowing temperature per a batch at manufacturing.

From the above matter, the layer thickness equal to or greater than 10nm is preferable for restraining quantum size effect. In addition, fromthe reason similar to the above matter, it is preferable that thicknessof each one of the second semiconductor layer is 10 to 50 nm.

From a view point of restraining a dual-peak electric field distributionshape, it is preferable to dispose first semiconductor layers and secondsemiconductor layers each having a plurality of thicknesses alternately.

In case where the first semiconductor layer is made from InP, in orderto make the total thickness of the second semiconductor layer equal toor smaller than 1 μm, it is preferable to make a period of repeating thefirst semiconductor layers and the second semiconductor layer 20 to 100periods.

Also, in case where the first semiconductor layer is made from GaInAsP,10 to 50 periods are preferable in order to make the total thickness ofthe second semiconductor layer equal to or smaller than 1 μm.

It should be noted that, although herein GaInAsP of which latticematching degree is −0.05% to 0.05% is used, a crystal having less defectcan be realized even when the total thickness of the GaInAsP is madeequal to or greater than 1 μm by a strain compensation structure inwhich the first semiconductor layer and the second semiconductor layerhave positive and negative lattice constants relative to latticeconstants of substrates, respectively. In this case, the period is notlimited. It should be noted that the respective total thicknesses of thefirst semiconductor layer and the second semiconductor layer are layerthicknesses equal to or smaller than critical thicknesses defined basedon lattice mismatching degree.

Also, preferable compositions for the first semiconductor layer and thesecond semiconductor layer are GaInAsP having band gaps which satisfythe following formulae.

That is, the first semiconductor layer satisfies 0≤E1/E0<0.35 and thesecond semiconductor layer satisfies 0.13≤E1/E0<0.71 where a differencebetween a band gap of the n-InP cladding layer 2 and a band gap of abarrier layer constituting the active layer 3 is EO (eV) and adifference between a band gap of the n-InP cladding layer 2 and a bandgap of a GaInAsP layer constituting the electric-field-control layer isE1 (eV). By using GaInAsP satisfying these conditions for theelectric-field-control layer, it is possible to drive a semiconductorlaser module of which optical output at an end of a fiber is equal to orgreater than 300 mW with low power consumption.

It should be noted that, although the semiconductor layer in theelectric-field-distribution-control layer 12 is configured so that bandgap energy is greater when leaving away from the GRIN-SCH-MQW activelayer 3, a configuration in which band gap energy is greater when beingcloser to the active layer 3 is applicable.

It should be noted that, although the semiconductor laser elementaccording to the embodiment 1 is a semiconductor laser element formed onthe InP substrate and uses the electric-field-distribution-control layer12 which is configured by the InP layer and the GaInAsP, anelectric-field-distribution-control layer configured by an InP layer andan AlGaInAsP layer may be used. Also, anelectric-field-distribution-control layer configured by a GaAs layer andan AlGaInAsP layer can be used in a semiconductor laser element formedon a GaAs substrate.

Next, a lower side SCH layer having 40.8 nm of layer thickness is formedby layering non-doped-GaInAsP layers having 0.95 μm, 1.0 μm, 1.05 μm,1.1 μm, and 1.15 μm of composing wavelengths. After that, a well layermade from GaInAsP and a barrier layer made from GaInAsP are grown on thelower side SCH layer alternately, and an active layer having a multiquantum well structure in which the number of well layers is three isformed. In the present embodiment 1, the layer thicknesses andcompositions of the well layer and the barrier layer are set so thatoscillation wavelength is 1415 nm. A compressive strained quantum wellstructure of which lattice mismatching degree relative to the InPsubstrate is approximately 1% is applied to the well layer.

It should be noted that, since net strain amount of strained quantumwell active layer can be reduced by introducing a strain compensationstructure having a barrier layer into which tensile strain isintroduced, a compressive strained quantum well structure of whichlattice mismatching degree of a quantum well layer is equal to orgreater than 1% is applicable.

In addition, in case where Zn is used as p-type impurity, since thediffusion coefficient of Zn is great, Zn diffuses in the active layer bya thermal process in manufacturing process (for example, growingtemperature etc. at regrowth when forming a BH structure), problems suchas decrease in optical output of the semiconductor laser element andincrease in threshold current occur, and in particular, in a pumpinglight source for use of a fiber amplifier, the decrease in opticaloutput is a problem. To address this, the present embodiment 1 has astructure in which Zn is suppressed to diffuse into the active layer bydoping the active layer with 0.3 to 1×10¹⁸/cm³ of n-type impurity.

Next, an upper side SCH layer having 40.8 nm of layer thickness isformed on the active layer by layering non-doped-GaInAsP having 0.95 μm,1.0 μm, 1.05 μm, 1.1 μm, and 1.15 μm of composing wavelengths. The abovedescribed lower side SCH layer, the active layer of the multi quantumwell structure, and the upper side SCH layer constitute the GRIN-SCH-MQWactive layer 3.

After that, a lower layer portion of the p-InP cladding layer, which ismade of InP of which impurity density is 3 to 9×10¹⁷/cm³ and of whichthickness is 0.5 μm is grown on the upper side SCH layer.

Herein FIG. 3 is a drawing showing a band diagram around theGRIN-SCH-MQW active layer 3 of the semiconductor laser element accordingto the embodiment 1. A well layer 3 a and a barrier layer 3 b arelayered alternately and sandwiched by a lower-side SCH layer 3 c and anupper side SCH layer 3 d. Layer thicknesses and compositions of thelower-side SCH layer 3 c and the upper side SCH layer 3 d which areGRIN-SCH and GaInAsP constituting the outermost barrier layer 3 b areset so that band gaps of respective layers are disposed linearly asshown by dotted lines L1, L2, L3, and L4. This is because efficiency ofinjecting carriers into the active layer is increased by realizing aquasi-linear SCH structure. It should be noted that there is no problemsince, even if it is not linearly, carriers can be injected efficientlyinto the active layer as long as the dotted lines L1 and L2 form a banddiagram having an upwardly projecting shape and the dotted lines L3 andL4 form a band diagram having a downwardly projecting shape and the twoprojecting shapes face each other. Also, if it is possible to controlcompositions strictly, it is no problem to use a linear SCH structure inwhich a III group element and a V group element which are compositionelements vary continuously.

It should be noted that, as an crystal-growing method other than MOCVDmethod, molecular beam epitaxy (MBE) method or chemical beam epitaxy(CBE) method may be used.

After that, an SiNx layer is deposited on the entire surface by thethickness of approximately 120 nm by plasma CVD method etc. and formedinto a stripe shape by a photolithography process to form an etchingmask, and a mesa shape is formed by being immersed in wet etchingsolution and the etching mask being used as an etching mask, so that acurved face is obtained and no particular plane orientation is exposed.In this state, the mesa is formed so that theelectric-field-distribution-control layer 12 remains with the thicknessof approximately 1 μm in a region in which the p-InP current block layer8 and the n-InP current block layer 9 are formed.

Although it is manufactured by using wet etching in the presentmanufacturing method, there is no problem if a mesa is formed by aprocess of dry etching and subsequent wet etching in order to removing adamage layer formed by the dry etching. From a view point of uniformitywithin a surface of the active layer width, the latter process isdesirable.

Subsequently, the p-InP current block layer 8 and the n-InP currentblock layer 9 are layered by MOVPE method by making use of the SiNxlayer as a selective growth mask to bury two sides of the mesa, andafter that, the SiNx layer is removed.

After that, an upper layer portion of the p-InP cladding layer made fromInP of which impurity density is 5 to 7×10¹⁷/cm³ is grown on the entiresurface by the thickness of 3.5 μm, and furthermore, the p-GaInAsPcontact layer 7 made from GaInAsP of which impurity density isapproximately 5×10¹⁸/cm³ is grown by the thickness of 0.5 μm.

The p-InP cladding layer 6 is configured to have 4.0 μm of thicknesswhich is the sum of the lower layer portion of the p-InP cladding layerof which thickness is 0.5 μm and the upper layer portion of the p-InPcladding layer of which thickness is 3.5 μm.

Then, a p-side electrode 10 is formed on an upper surface of thep-GaInAsP contact layer 7 and an n-side electrode 11 is formed at alower side of the n-InP substrate 1, and after that, it is cut with alength of 2 mm and is given a reflection-side reflecting coating 14 anda output-side reflecting coating 15 to form a laser structure. By doingthis way, the semiconductor laser element according to the embodiment 1is finished.

FIGS. 4A and 4B are drawings showing stripe shapes in the waveguidedirection of light in the semiconductor laser element according to theembodiment 1 and a comparison element respectively. The comparisonelement is a conventional Fabry-Perot type semiconductor laser elementwhich is not provided with the electric-field-distribution-control layer12. Herein the comparison element has a layered structure identical withthe semiconductor laser structure according to the embodiment 1 exceptthat the thickness of the n-InP cladding layer 2 is 1.3 μm and thecomparison element does not include theelectric-field-distribution-control layer 12.

As shown in FIG. 4A, regarding the present embodiment 1, thesemiconductor laser element, in which the width of the GRIN-SCH-MQWactive layer 3 has an approximately 4.3 μm of active layer width alongthe waveguide direction of light, was manufactured.

Also, as shown in FIG. 4B, in order to control a single transverse mode,the comparison element was configured to have a waveguide structure oflight in which active layer width of the GRIN-SCH-MQW active layer alongthe waveguide direction of light is configured by linear regions (lengthof 30 μm: a light-output-side region, 750 μm: a light-reflecting sideregion) in which the active layer width is 2.7 μm and a region (length620 μm) which is provided with a linear active layer width which iswider than width in the vicinity of the light-output-end surface and issandwiched by regions (length 300 μm) varying in a taper manner. Itshould be noted that, the active layer width of the comparison elementis defined by a value obtained by dividing the area of the active layerby the length of the cavity. For example, the comparison element having4.3 μm of active layer width is realized by adjusting the taper regionand the linear active layer width which is wider than width in thevicinity of the light-output-end surface so that the area of the activelayer is 8.6 μm². It should be noted that, since the single transversemode can be maintained at 2.7 μm of active layer width, the structure ofthe comparison element was made a linear stripe structure. By doing thisway, it is a structure of controlled single transverse mode control andpreventing optical output saturation due to heat when injecting highelectric current.

FIG. 5 is a drawing showing current-optical output characteristics ofthe semiconductor laser element according to the embodiment 1 and acomparison element.

It is found that, although the semiconductor laser element according tothe present embodiment 1 and the comparison element have stripes having4.3 μm of width, in the characteristics of FIG. 5 , a kink which is adiscontinuous point is not generated and high power output operationequal to or greater than 800 mW is achieved.

This is caused by an effect of reducing a refractive index differencebetween a current-confining region and an active layer region becauseapproximately 1 μm of thickness of theelectric-field-distribution-control layer 12 remains in a region inwhich the p-InP current block layer 8 and the n-InP current block layer9 are formed.

Also, as shown in FIG. 5 , when driving current is 1800 mA, the opticaloutput of the semiconductor laser element according to the embodiment 1shows power output higher than the optical output of the comparisonelement by approximately 70 mW. This is because inter valence bandabsorption in the p-InP cladding layer 6 reduced and external quantumefficiency increased along with reduction in internal loss since theelectric-field-distribution-control layer 12 is applied to thesemiconductor laser element of the present embodiment 1.

From the above description, although a complex waveguide structure asshown in FIG. 4 was applied to the comparison element in order tocontrol the single transverse mode, the semiconductor laser structureaccording to the present embodiment 1 obtains an effect of enablingsingle transverse mode control with a simple structure and realizing asemiconductor laser with efficiency higher than that of the comparisonelement by applying the electric-field-distribution-control layer 12.

FIG. 6 is a drawing showing dependences of electric input power onoptical output of the semiconductor laser element according to theembodiment 1 and the comparison element. As compared with the comparisonelement, the semiconductor laser element according to the presentembodiment 1 has an effect of being able to drive over a wide range froma low power output side to 800 mW of high power output with low powerconsumption.

It should be noted that it is needless to say that a high power outputpumping light source for use of a Raman amplifier at 1400 nm to 1550 nmcan be realized with low power consumption by optimizing thickness orcomposition of the quantum well active layer in the semiconductor laserelement according to the present embodiment 1.

Also, although the present embodiment 1 is a linear mesa-stripestructure in which the width of the active layer is equal with respectto the direction of propagation of light, a structure maintaining thesingle transverse mode in which stripe width varies with respect to thedirection of propagation of light, such as, for example, a taperstructure or an active MMI structure may be used. Also, since the activelayer width can be equal to or greater than 3 μm in the presentembodiment 1, in case where it is applied to the comparison elementstructure, the active layer width of the linear region of thelight-output end face can be expanded, and the area of the active layercan be increased to a greater extent.

As a result, element resistance can be reduced, and a lowpower-consuming semiconductor laser element can be realized.

Although the present embodiment 1 is a semiconductor laser elementhaving a BH structure, the present invention can be applied to aridge-stripe type laser or an SAS type laser easily.

Also, although the present embodiment 1 is a semiconductor laserelement, the present invention can be applied to various aspects of awavelength-variable laser, for example, an MOPA structure, asemiconductor optical amplifier, or an element, in which opticalfunctions are integrated, such as a modulator-integrated laser orwavelength-variable laser in which a plurality of functions areintegrated on a same substrate. The functions include to select aspecific wavelength by returning (feedbacking) a specific wavelength inthe wavelength-variable laser. Such wavelength-variable laser includes,for example, a Distributed FeedBack (DFB) laser.

To enable a high-output DFB laser oscillating in a single mode, it isformed in a laser element a diffracting grating that obtains a productκLg of a coupling coefficient κ of the diffracting grating and a lengthLg of the diffracting grating from 0.6 to 1.0, more preferably, from 0.7to 0.8. It is formed, more preferably, on the front end surface adielectric reflection film having a reflectance less than that of theback end surface to obtain an optical output efficiently.

Although a dielectric reflection film is used herein, a reflectionstructure including a semiconductor diffracting grating may be used.

The maximum of Lg is the element length L, and the diffracting gratingis formed for a part of or the entire length of the laser element. Theelement length L is 300 μm or longer and, preferably, 1000 μm or longer.In light of low-cost manufacture, the element length L is preferably5000 μm or shorter.

The diffracting grating is preferably formed on a side of the activelayer opposite to a side where the electric-field adjusting layer is.This is because if the diffracting grating and the electric-fieldadjusting layer are on the same side, the optical confinement of theactive layer vary because of production tolerance during the process offorming the diffracting grating layer, which causes difficulty inmaintaining a high manufacturing yield. In addition, with thisstructure, the optical confinement of the diffracting grating is less ascompared to when the diffracting grating and the electric-fieldadjusting layer are on the same side, and the above-mentioned κ can beachieved with a good reproducibility.

Moreover, to improve operating characteristics under a high temperature,the number of strained quantum well layers of the active layer formed ispreferably six or more. A reason therefor is explained below. Arelated-art laser structure having no electric-field adjustment aims toachieve low loss for purpose of high output and achieves the aim bydecreasing the number of well layers or decreasing p-type impurity.This, however, makes temperature characteristics poor due to an impactof heat generated by carrier overflow or an increase in the drivingvoltage.

Because a strained super lattice is used in the quantum well activelayer, the thickness of the entire active layer is adjusted to a valueequal to or less than a critical film thickness such that no dislocationoccurs in the crystal. To use an active layer having higher strain, itis allowable to employ such structure in which a strained super latticethat cancels strain of the well layers is introduced to the barrierlayer so that the amount of strain in the entire active layer becomeszero.

Although it is described herein about a DFB laser oscillating with ahigh-output single mode, the laser selecting a particular wavelength maybe a laser element oscillating with multiple modes. To achieve this, κLgis preferably set to 0.3 or less. When the diffracting grating is to beformed for the entire element length, this can be achieved by usage of ahigh-order diffracting grating.

In light of selecting a particular wavelength, not only a DFB laser butalso a DR laser having a wavelength selectable reflection structure onat least one of the front end surface or the back end surface areallowable.

As semiconductor laser elements according to embodiments 2-1 and 2-2 ofthe present invention, a semiconductor laser element in which theconfiguration of an electric-field-distribution-control layer 12 isdifferent from the embodiment 1 is manufactured, and lasercharacteristics thereof were compared with the semiconductor laserelement according to the embodiment 1 and the comparison element.

The embodiment 2-1 is a semiconductor laser element having anelectric-field-distribution-control layer 12 which is configured by 8periods of a GaInAsP layer of which composing wavelength is 0.95 μm andthickness is 20 nm, an InP layer of which thickness is 180 nm, a GaInAsPlayer of which composing wavelength is 1.1 μm and thickness is 20 nm,and an InP layer of which thickness is 180 nm, and the total thicknessof the electric-field-distribution-control layer 12 is 1800 nm. Also,the embodiment 2-2 is a semiconductor laser element having anelectric-field-distribution-control layer 12 which is configured by 4periods of a GaInAsP layer of which composing wavelength is 0.95 μm andthickness is 20 nm, an InP layer of which thickness is 180 nm, a GaInAsPlayer of which composing wavelength is 1.0 μm and thickness is 100 nm,and an InP layer of which thickness is 300 nm, and 4 periods of aGaInAsP layer of which composing wavelength is 1.0 μm and thickness is40 nm and an InP layer of which thickness is 360 nm, and the totalthickness of the electric-field-distribution-control layer 12 is 3400nm.

In the embodiment 2-1, the total thickness of the GaInAsP layer includedin the electric-field-distribution-control layer 12 is 180 nm which is10.0% of the total thickness (1800 nm) of theelectric-field-distribution-control layer 12. In the embodiment 2-2, thetotal thickness of the GaInAsP layer is 580 nm which is 17.1% of thetotal thickness (3400 nm) of the electric-field-distribution-controllayer 12.

Optical confinement coefficients of the active layers by calculation are0.9% in the embodiment 1, 0.97% in the embodiment 2, and 0.91% in theembodiment 2-2, and as compared with 1.1% in the comparison element,they are structures in which confinement is slightly small.

Also, values of internal losses are 2.7/cm in the embodiment 1, 2.5/cmin the embodiment 2, and 2.4/cm in the embodiment 2-2 which areapproximately the same and estimated to be values smaller than 3.8/cm inthe comparison element, and a possibility that external differentialquantum efficiency may be enhanced due to reduction in inter valenceband absorption can be expected.

When IL kink (discontinuous point of optical output) is generated in asemiconductor laser element, a high-order lateral mode is generated in acurrent injection region following the point where the kink isgenerated.

This causes a problem that, when laser light is coupled to an opticalfiber via a lens, coupling efficiency deteriorates to a great extent,and a high power output semiconductor laser module cannot be realized.

In addition, although an optical fiber amplifier is used by controllingdriving current automatically so that optical output is constant(APC-driven), there is a problem that APC control is not possible whenthere is IL kink. From the above matter, it is important that a kink isnot generated within a range of driving current of a pumping lightsource for use in an optical fiber amplifier.

FIG. 7 is a drawing showing a relationship between an active layer widthand a rate of occurrence of IL kink of the semiconductor laser elementaccording to the embodiments 1, 2-1, and 2-2. Rate of occurrence of ILkink increases when the active layer width is greater than 4.3 μm in thesemiconductor laser element according to the embodiment 1 or when theactive layer width is greater than 3.3 μm in the semiconductor laserelement according to the embodiment 2-1. In contrast to this, it isfound that, in the semiconductor laser element according to theembodiment 2-2, the rate of occurrence of the IL kink is restrained evenwhen the active layer width is 4.8 μm.

Herein, in the embodiment 1, the thickness (remaining thickness) of theelectric-field-distribution-control layer 12 remaining in a lower layerof the p-InP current block layer 8 constituting the BH structure forconfining current is 1 μm (0.95 μm composition wavelength GaInAsP/InP=20nm/180 nm, 1 μm composition wavelength GaInAsP/InP=(60 nm/140 nm)×4periods) among which the thickness of the GaInAsP layer is 260 nm.

On the other hand, in the embodiment 2-2, the remaining thickness of theelectric-field-distribution-control layer 12 in the lower layer of thep-InP current block layer 8 is 1.2 μm (0.95 μm composition wavelengthGaInAsP/InP=20 nm/180 nm, 1 μm composition wavelength GaInAsP/InP=(100nm/300 nm)×4 periods) among which the thickness of the GaInAsP layer is420 nm. Similarly, in the embodiment 2-1, the remaining thickness of theelectric-field-distribution-control layer 12 is 0.2 μm (0.95 μmcomposition wavelength GaInAsP/InP=20 nm/180 nm) among which thethickness of the GaInAsP layer is 20 nm.

From the above matter, it is found that, if the remaining thickness ofthe electric-field-distribution-control layer 12 is greater, thethickness occupied by the GaInAsP layer increases, refractive indexdifference between the BH region and the active layer region is reduced,and high power output operation can be realized without generating akink even if the active layer width is wide. It should be noted that, ifthe remaining thickness is equal to or greater than 0.2 μm, generationof a kink can be restrained even if the active layer width is wide,i.e., equal to or greater than 3 μm.

FIGS. 8 to 11 are drawings showing dependence of static characteristicsof the semiconductor laser element on the active layer width. FIG. 8 isa drawing showing dependence of threshold current on the active layerwidth of each of the semiconductor laser elements according to theembodiments 1, 2-1, and 2-2 and the comparison element.

As shown in FIG. 8 , the threshold current of the semiconductor laserelement according to the embodiments 1, 2-1, and 2-2 exhibits a greatvalue as compared with that of the comparison element. As compared with4.3 μm of the active layer width, it is greater by approximately 5 mA.

This is because, by introducing the electric-field-distribution-controllayer 12, an electric field is distributed at n-InP substrate side andconfinement of light decreases in the active layer.

Comparison among semiconductor laser elements according to theembodiments 1, 2-1, and 2-2 shows tendency that difference amongthreshold current of the respective embodiments decreases along withincrease in the active layer width. In addition, in the embodiment 1 orthe embodiment 2-2, threshold current increases when the active layerwidth is smaller than a certain active layer width.

This is considered because density of carriers injected into the activelayer increases along with decrease in the active layer width,non-radiative recombination component in theelectric-field-distribution-control layer 12 increases, and efficiencyof injecting current into the active layer is reduced. In particular, itis found that, along with increase in the total thickness of the GaInAsPlayer included in the electric-field-distribution-control layer 12, thatis, increase in the threshold current in a region in which active layerwidth is small becomes greater in the structure of the embodiment 2-2than in the structure of the embodiment 1. The increase in thresholdcurrent caused by this increase in the total thickness of the GaInAsPlayer can be avoided by reducing carrier density by setting the activelayer width at equal to or greater than 3.3 μm, more preferably, equalto or greater than 4.3 μm.

FIG. 9 is a drawing showing dependence of optical output (when drivingat 1.8 A) on the active layer width of each of the semiconductor laserelement according to the embodiments 1, 2-1, and 2-2 and the comparisonelement. In the semiconductor laser elements according to the embodiment1 and the embodiments 2-1 and 2-2, higher power output than opticaloutput of the comparison element by approximately 30 to 70 mW isachieved. This is also effect caused by the above described reduction inthe inter valence band absorption. It should be noted that, in case ofthe comparison element, output is approximately 660 mW to 680 mW at 2.7μm to 5.1 μm of the active layer width.

It is found that, among the structures of the three embodiments, opticaloutput is the greatest within a range of 2.7 μm to 4.2 μm of the activelayer width in the structure of the embodiment 1 in which theelectric-field-distribution-control layer 12 has an intermediate layerthickness, and there is an optimum value in the layer thickness of theelectric-field-distribution-control layer 12.

That is, it is found that, since there is a trade-off relationshipregarding the above described single transverse mode control and highpower output operation, optimization of theelectric-field-distribution-control layer 12 is important to realize anoperation with low power consumption and high power output in thesemiconductor laser element having theelectric-field-distribution-control layer 12.

It was found that, in order to realize 99.7% of yield rate at which thesemiconductor laser module of which fiber-end optical output is 500 mWis operated with driving current which is equal to or lower than 10 W,optical output which is equal to or greater than 725 mW is necessarywhen driving at 1.8 A. The optimum range of the total thickness of theelectric-field-distribution-control layer 12 obtained from this findingand the results of the respective embodiments in which the active layerwidth of is 3.9 μm in FIG. 9 is 1.8 μm to 3.5 μm.

FIG. 10 is a drawing showing dependence of driving voltage (when drivingat 1.8 A) on the active layer width of each of the semiconductor laserelements according to the embodiments 1, 2-1, and 2-2 and the comparisonelement. FIG. 11 is a drawing showing dependence of differentialresistance (when driving at 1.8 A) on the active layer width of each ofthe semiconductor laser elements according to the embodiments 1, 2-1,and 2-2 and the comparison element. In either one of the FIGS. 10 and 11, comparison of driving voltage and differential resistances between thesemiconductor laser elements according to the embodiments 1, 2-1, and2-2 and the comparison element shows similar value at a same activelayer width. It should be noted that, at 2.7 μm to 5.1 μm of the activelayer width, similar values to those of the semiconductor laser elementaccording to the embodiments 1, 2-1, and 2-2 are shown. From this, it isfound that, the electric-field-distribution-control layer 12 having aplurality of hetero interfaces to the extent examined in the presentembodiment is characterized in that resistance is small and does notaffect electric characteristics.

A semiconductor laser module according to an embodiment 3 of the presentinvention will be explained.

The semiconductor module according to the present embodiment 3 uses thesemiconductor laser element, according to the embodiment 1, having 4.3μm of active layer width.

FIG. 12 is a side cross section showing the structure of a semiconductorlaser module according to the embodiment 3 of the present invention. Thesemiconductor laser module according to the present embodiment 3 isprovided with a semiconductor laser element 52 which corresponds to thesemiconductor laser element according to the above describedembodiment 1. It should be noted that this semiconductor laser element52 has a junction-down configuration in which a p-side electrode isjunctioned to a laser mount 48. A temperature-control module 50 as atemperature-control device is disposed on an internal bottom surface ofa package 51 which is a casing of the semiconductor laser moduleaccording to the present embodiment 3 and is formed by ceramic etc.

A base 47 is disposed on the temperature-control module 50, and thelaser mount 48 is disposed on the base 47. Current is given to thetemperature-control module 50 for cooling and heating according to itspolarity. It should be noted that it functions mainly as a cooler forpreventing oscillation wavelength shift due to increase in temperatureof the semiconductor laser element 52. That is, the temperature-controlmodule 50 cools the semiconductor laser element 52 and controls to lowertemperature in case where wavelength of the laser light is longer than adesirable wavelength, and heats the semiconductor laser element 52 andcontrols to higher temperature in case where wavelength of the laserlight is shorter than a desirable wavelength

Specifically, this controlled temperature is controlled based on valuesdetected by a thermistor 49 disposed on the laser mount 48 and in thevicinity of the semiconductor laser element 52. A control device notshown in the drawings usually controls the temperature-control module 50so that the temperature of the laser mount 48 is maintained constant. Inaddition, the control device not shown in the drawings controls thetemperature-control module 50 so that temperature of the laser mount 48decreases along with increase in driving current of the semiconductorlaser element 52. By performing such temperature control, stability ofoutput from the semiconductor laser element 52 can be enhanced and it iseffective for enhancing product yield. It should be noted that it isdesirable that the laser mount 48 is formed by material, for examplediamond, having high thermal conductivity. This is because generation ofheat when applying high electric current is restrained by forming thelaser mount 48 with diamond.

The laser mount 48 on which the semiconductor laser element 52 and thethermistor 49 are disposed, a first lens 53 which is a optical-couplinglens system, and a light-receiving element 46 for monitoring light aredisposed on the base 47. Laser light outputted from the semiconductorlaser element 52 is guided to an optical fiber 45 via the first lens 53,an isolator 54, and a second lens 44 which is an optical-coupling lenssystem. The second lens 44 is disposed on an optical axis of the laserlight and on the package 51 and coupled optically with the optical fiber45 connected externally thereto. The optical fiber 45 guides the laserlight outside thereto. It should be noted that the light-receivingelement 46 for monitoring light is for monitoring and detecting lightleaking from a high reflection film side of the semiconductor laserelement 52 and measuring optical output of the semiconductor laserelement 52.

Herein in this semiconductor laser module, an optical isolator 54, whichtransmits the laser light outputted from the semiconductor laser element52 at the second lens 44 side, is disposed between the semiconductorlaser element 52 and the optical fiber 45 so that reflect return lightby other optical component does not return to inside the cavity. Itshould be noted that, although the isolator 54 of which extinction ratiois −20 dB is used herein, it is preferable to use an isolator equal toor lower than −20 dB in order to restrain reflection from componentsconstituting the module or the system. It should be noted that powerconsumption of the semiconductor laser module according to the presentembodiment was 9 W when optical output at the fiber end was 500 mW.Since, in a laser module using the comparison element of which activelayer width was 4.3 μm, power consumption was 11.6 W when optical outputat the fiber end was 500 mW, approximately 23% of reduction in powerconsumption can be realized.

In addition, in case where the semiconductor laser element 52 isconstituted by the structure shown in FIGS. 1 and 2 , a structure isemployed in which a fiber grating as an optical feedback unit returninga part of laser light propagating in the optical fiber 45 to thesemiconductor laser element 52 is disposed in the optical fiber 45, anda cavity is formed together with the reflection side of an end surfaceof the semiconductor laser element 52. In this case, the opticalisolator 54 should not be disposed in the semiconductor laser module butmust be of an inline type in which the optical isolator 54 is disposedat a stage subsequent to the fiber grating.

As a modification example of the embodiment 3, a semiconductor lasermodule which does not include an optical isolator in the configurationof the embodiment 3 but has a fiber grating disposed at a part of theoptical fiber will be explained. In the modification example accordingto the present embodiment, a semiconductor laser module having a fibergrating (FBG) is configured by using the semiconductor laser elementaccording to the embodiment 1. It should be noted that characteristicswere compared by manufacturing a semiconductor laser module which isprovided with a fiber grating by using the comparison element.

It should be noted that characteristics of the FBG used formanufacturing the module are 1.8% of reflectivity, 1.8 nm of reflectionband width (full width at half maximum), and 1425 nm of centralwavelength.

In the semiconductor laser module using the comparison element, thecomparison element described in FIG. 4 and having 4.3 μm of active layerwidth was used.

In the semiconductor laser according to the embodiment 1, far-fieldpattern was 11.8° and 16° in the horizontal direction and the verticaldirection respectively, and in the comparison element, far-field patternwas 12.5° and 19.6° in the horizontal direction and the verticaldirection respectively.

Since more round beam can be realized than the comparison element by thesemiconductor laser element according to the embodiment 1 of the presentinvention, higher efficiency of coupling with an optical fiber ispossible. By reduction of driving current and driving voltage by this,an effect of reducing power consumption of the module is obtained.

FIG. 13 is a view showing a relationship between optical output at anend of a fiber and driving current of a semiconductor laser module usingthe element according to the embodiment 1 or the comparison element.Since, in the semiconductor laser module using the element according tothe embodiment 1, driving current when optical output at the fiber endis 500 mW is reduced from that of the semiconductor laser module usingthe comparison element, the above described effect of reducing the powerconsumption can be confirmed. It should be noted that couplingefficiency for the semiconductor laser module using the elementaccording to the embodiment 1 was confirmed to be 82% and was a valuegreater than 72% of coupling efficiency for the semiconductor lasermodule using the comparison element. This is an effect caused byfar-field pattern of the element according to the embodiment 1 beingvaried closer to a more round beam.

FIG. 14 is a view showing a relationship between optical output andpower consumption at the end of the fiber of the semiconductor lasermodule using the element according to the embodiment 1 or the comparisonelement. In the semiconductor laser module using the element accordingto the embodiment 1, power consumption at 500 mW-optical output at thefiber end is reduced from power consumption of the semiconductor lasermodule using the comparison element by approximately 2.7 W. In addition,since optical output at the fiber end was 580 mW in case where, in thesemiconductor laser module using the element according to the embodiment1, power consumption was set at power consumption that is the same aswhen optical output was set at 500 mW at the fiber end of thesemiconductor laser module using the comparison element, 16% ofimprovement is achieved in optical output.

In addition, although power consumption was 16 W when the maximumoptical output of the semiconductor laser module using the comparisonelement was 560 mW, in the semiconductor laser module according to thepresent embodiment, fiber end optical output which is equal to orgreater than 600 mW is obtained at approximately 12 W of powerconsumption.

From the above matter, there is an effect that high power optical outputcan be realized with low power consumption by using the semiconductorlaser element according to the embodiment 1 in the semiconductor lasermodule. Also, it was found experimentally that, when reflectivity atoutput side of the laser light is equal to or lower than 0.5%, morepreferably equal to or lower than 0.2%, generation of a kink at highpower output can be restrained, and a pumping light source for fiberamplification in which dynamic range is wide from a low output region toa high output region with high power output and stabilized wavelengthcan be realized.

FIG. 15 is a view showing a relationship between module powerconsumption and thermistor temperature of the semiconductor laser moduleusing the element according to the embodiment 1 or the comparisonelement at a time of 500 mW-optical output at the fiber end.

FIG. 15 shows power consumption when the semiconductor laser element wasoperated at high temperature.

Herein evaluation was performed by varying thermistor temperature of themodule in order to vary temperature of the semiconductor laser element.As a result, although, in case where thermistor temperature was 25° C.,power consumption of the semiconductor laser module was approximately 8W, the power consumption can be reduced to 6.2 W by driving thesemiconductor laser element at 40° C. In addition, in the semiconductorlaser module using the element according to the embodiment 1, powerconsumption is reduced by approximately 2.7 W relative to thesemiconductor laser module using the comparison element.

This is because load to the temperature-control module 50 which is anelectronic cooler was reduced by increasing temperature of thethermistor, and power consumption of the temperature-control module 50decreased.

Therefore, by using the semiconductor laser element according to thepresent embodiment 1 for the semiconductor laser module, thesemiconductor laser element can be driven at approximately 40° C. ofhigh temperature, and optical output of 500 mW at the fiber end can beobtained by being driven at approximately 6 to 8 W of low powerconsumption. As a result, there is an effect that power consumption ofthe optical fiber amplifier can be reduced by using this semiconductorlaser module for the optical fiber amplifier.

As explained above, the semiconductor laser module according to theembodiment 3 uses the semiconductor laser element according to theembodiment 1 for the semiconductor laser element 52. Since thesemiconductor laser element 52 has a feature of being operable with lowpower consumption and high power output and being capable of couplingwith the optical fiber via the optical component with high couplingefficiency, a semiconductor laser module with low power consumption andhigh power output can be realized. Furthermore, since the semiconductorlaser element 52 has a feature of high photoelectric conversionefficiency, the semiconductor laser element 52 can be operated at hightemperature, and thus load to the temperature-control module 50 isreduced, a semiconductor laser module with lower power consumption andhigh power output can be realized.

It should be noted that, although the 2-lens system is employed in thepresent embodiment, a semiconductor laser module of a 1 lens system maybe configured by components such as the semiconductor laser elementaccording to the embodiment of the present invention, a focusing lens,an isolator, and an optical fiber etc. Also, a semiconductor lasermodule can be configured by the semiconductor laser element according tothe embodiments of the present invention and a lensed fiber. Inparticular, in a semiconductor laser module having a fiber grating whichis an optical fiber having a grating provided to a part thereof, thereis an effect that laser oscillation wavelength is stabilized at awavelength selected by the fiber grating.

It should be noted that a butterfly package is used in the presentembodiment 3, a semiconductor laser module using a Can type package maybe configured.

It should be noted that, as described above, in the modification exampleof the present embodiment 3, when the FBG of which reflectivity is 1.8%,reflection band width (full width at half maximum) is 1.8 nm, andcentral wavelength is 1425 nm was used, the power consumption whenoptical output at the fiber end was 500 mW was 8 W. In contrast to this,when using an FBG of which reflectivity is 3.5% and othercharacteristics are the same as those of the above description, thepower consumption when optical output at the fiber end was 500 mW was8.5 W.

It should be noted that, a Raman amplifier has a problem that, ifspectral line width of a pumping light source is narrow, sufficientRaman gain cannot be obtained because of influence of stimulatedBrillouin scattering which is non-linear effect in an optical fiber.

As a countermeasure to this, it is effective to include a great numberof Fabry-Perot modes within reflection band width equal to or greaterthan 1 nm of an FBG in spectrum of laser light. For that purpose, it iseffective to make a cavity of a semiconductor laser element longer orextend the reflection band width of the FBG.

For example, in the present modification example, the laser element ofwhich cavity length is 2 mm includes 12 longitudinal modes of whichinterval is approximately 0.15 nm within reflection band width.Furthermore, by setting the cavity length at 3 mm, longitudinal modeinterval becomes 0.1 nm, thus it is possible to include 18 longitudinalmodes, which is 1.5 times the above described case, within thereflection band width of the FBG.

As other methods, a module configuration, in which 2 FBGs of whichwavelengths are substantially the same are disposed at an interval of 80cm to 100 cm for collapsing coherence of laser light, is effective forrestraining influence of stimulated Brillouin scattering. In this state,the FBG closer to the laser element is disposed so that distance fromthe laser chip is equal to or greater than 80 cm.

It was confirmed that, in a semiconductor laser module using the elementof according to the embodiment 1 (4.3 μm of active layer width)manufactured with this configuration, there is not influence ofstimulated Brillouin scattering when optical output is equal to orgreater than 50 mW.

Furthermore, in the above described method, a problem occurs in anaspect of cost because 2 FBGs are used. For the purpose of reducingcost, in order to collapse coherence of laser light with one FBG andrestrain influence of stimulated Brillouin scattering, it is effectiveto equalize reflectivity of the laser end surface and reflectivity ofthe FBG substantially.

In a semiconductor laser module using the element (active layer width is4.3 μm, reflectivity at laser output end surface is 1%) of theembodiment 1 manufactured with this configuration, it is confirmed thatinfluence of stimulated Brillouin scattering is restrained when opticaloutput was equal to or greater than 100 mW and FBG reflectivity is1.8%±0.36%, or when optical output was equal to or greater than 50 mWand FBG reflectivity is 1.0%±0.2%. It should be noted that, forcompatibly achieving higher power output and restraining influence ofstimulated Brillouin scattering, it is preferable that reflectivity atlaser output end surface is 1% and FBG reflectivity is 1.0%±0.2%.

It should be noted that, since a pumping light source for use of Ramanamplifier is usually used by multiplexing wavelengths of laser lightfrom a plurality of pumping light sources, it is preferable to setreflecting band of an FBG at equal to or lower than 2 nm inconsideration of loss of optical multiplexer when multiplexingwavelengths.

An optical fiber amplifier according to an embodiment 4 of the presentinvention will be explained. The embodiment 4 is characterized in thatthe semiconductor laser module of the above described embodiment 3 isapplied to a Raman amplifier.

FIG. 16 is a block diagram showing a configuration of an optical fiberamplifier which is a Raman amplifier according to the embodiment 4. ThisRaman amplifier is used for a WDM communication system. In FIG. 16 , theRaman amplifier according to the embodiment 4 is configured to usesemiconductor laser modules 60 a to 60 d of which configurations are thesame as the semiconductor laser module shown in the above describedembodiment 3. It should be noted that, as a semiconductor laser elementto be used for the semiconductor laser module, those shown in theembodiments 1, 2-1, and 2-2 can be used.

Each of the semiconductor laser modules 60 a and 60 b outputs laserlight having a plurality of oscillation longitudinal modes to apolarization combining coupler 61 a via a polarization-maintainingoptical fiber 71, and each of the semiconductor laser modules 60 c and60 d outputs laser light having a plurality of oscillation longitudinalmodes to a polarization combining coupler 61 b via thepolarization-maintaining optical fiber 71. Herein wavelengths of thelaser light at which the semiconductor laser modules 60 a and 60 boscillate are the same. In addition, although wavelengths of the laserlight at which the semiconductor laser modules 60 c and 60 d oscillateare the same, but are different from the wavelengths of the laser lightat which the semiconductor laser modules 60 a and 60 b oscillate. SinceRaman amplification has dependence on polarization, it is configured sothat laser light of which dependence on polarization is canceled by thepolarization combining couplers 61 a and 61 b is outputted.

Laser light outputted by each of the polarization combining couplers 61a and 61 b and having different wavelengths are multiplexed by the WDMcoupler 62. The multiplexed laser light is outputted as pumping lightfor Raman amplification to an amplification optical fiber 64 via the WDMcoupler 65. Signal light to be amplified is inputted to theamplification optical fiber 64, into which the pumping light isinputted, and Raman amplification is performed.

The signal light (amplification signal light) having performed Ramanamplification in the amplification fiber 64 is inputted to amonitor-light-distributing coupler 67 via the WDM coupler 65 and anoptical isolator 66. The monitor-light-distributing coupler 67 outputs apart of the amplification signal light to a control circuit 68 andoutputs the rest of the amplification signal light to thesignal-light-outputting optical fiber 70 as output laser light.

The control circuit 68 controls laser-outputting state, for example,optical power of each of the semiconductor laser modules 60 a to 60 dbased on the inputted part of the amplification signal light andperforms feedback control so that characteristics of a gain band ofRaman amplification becomes flat.

As described above, since the optical fiber amplifier according to thepresent embodiment 4 configures a Raman amplifier by using thesemiconductor laser module 60 a in which the semiconductor laser elementaccording to the embodiment 1, 2-1, or 2-2 is built in, power of laserlight outputted from the semiconductor laser module can be enhanced.

FIG. 17 is a block diagram showing a modification example 1 of theoptical fiber amplifier according to the embodiment 4. Although thepolarization combining couplers 61 a and 61 b are used in the Ramanamplifier shown in FIG. 16 , light may be outputted to the WDM coupler62 directly from the semiconductor laser modules 60 a and 60 c throughthe polarization-maintaining fiber 71 respectively. In this case,polarization planes of the laser light outputted from the semiconductorlaser modules 60 a and 60 c are incident at an angle of 45 degreesrelative to the optical axis of the polarization-maintaining opticalfiber 71.

In addition, the Raman amplifier shown in FIGS. 16 and 17 is of abackward pumping type, Raman amplification can be performed stably evenif it is of forward pumping type or bi-directional pumping type.

For example, FIG. 18 is a block diagram showing a configuration of amodification example 2 of the optical fiber amplifier according to theembodiment 4, which is a Raman amplifier adopting forward pumping type.The Raman amplifier shown in FIG. 18 has a WDM coupler 65 a, in place ofthe WDM coupler 65 in the Raman amplifier shown in FIG. 16 , in thevicinity of the optical isolator 63. A circuit having semiconductorlaser modules 60 aa to 60 da, polarization combining couplers 61 aa and61 ba, and a WDM coupler 62 a corresponding respectively to thesemiconductor laser modules 60 a to 60 d, the polarization combiningcouplers 61 a and 61 b, and the WDM coupler 62 is connected to this WDMcoupler 65 a, and forward pumping is performed in which pumping lightoutputted from the WDM coupler 62 a is outputted in the direction sameas that of the signal light.

Similarly, FIG. 19 is a block diagram showing a configuration of amodification example 3 of the optical fiber amplifier according to theembodiment 4 which is a Raman amplifier adopting forward pumping type.The Raman amplifier shown in FIG. 19 has the WDM coupler 65 a, in placeof the WDM coupler 65 in the Raman amplifier shown in FIG. 17 , in thevicinity of the optical isolator 63. A circuit having the semiconductorlaser modules 60 aa and 60 c′ and the WDM coupler 62 a correspondingrespectively to the semiconductor laser modules 60 a and 60 c and theWDM coupler 62 is connected to this WDM coupler 65 a, and forwardpumping is performed in which pumping light outputted from the WDMcoupler 62 a is outputted in the direction same as that of the signallight.

Also, FIG. 20 is a block diagram showing a configuration of amodification example 4 of the optical fiber amplifier according to theembodiment 4, which is a Raman amplifier adopting bi-directional pumpingtype. The Raman amplifier shown in FIG. 20 is provided with the WDMcoupler 65 a, the semiconductor laser modules 60 aa to 60 da, thepolarization combining coupler 61 aa and 61 ba, and the WDM coupler 62 ashown in FIG. 18 in addition to the configuration of the Raman amplifiershown in FIG. 16 , and is of bi-directional pumping type in whichbackward pumping and forward pumping are performed.

Similarly, FIG. 21 is a block diagram showing a configuration of amodification example 5 of the optical fiber amplifier according to theembodiment 4 which is a Raman amplifier adopting bi-directional pumpingtype.

The Raman amplifier shown in FIG. 21 is provided with the WDM coupler 65a, the semiconductor laser modules 60 aa and 60 ca, and the WDM coupler62 a shown in FIG. 19 in addition to the configuration of the Ramanamplifier shown in FIG. 17 , and is of bi-directional pumping type inwhich backward pumping and forward pumping are performed.

The above described Raman amplifier shown in FIGS. 16 to 21 can beapplied to a WDM communication system. FIG. 22 is a block diagramshowing a schematic configuration of a WDM communication system to whichthe optical fiber amplifier is applied which is a Raman amplifieraccording to the embodiment 4 or the modification example thereof.

In FIG. 22 , optical signals, of which wavelengths are λ1 to λn,transmitted from a plurality of transmitters Tx1 to Txn are multiplexedby an optical multiplexer 80 and coupled to an optical fiber 85. Aplurality of Raman amplifiers 81, 83 corresponding to the Ramanamplifier shown in FIGS. 16 to 21 are disposed on a transmission path ofthe optical fiber 85 according to transmission distance and amplifiesthe optical signal which is attenuated. The optical signal transmittedon the optical fiber 85 is demultiplexed by an optical demultiplexer 84into a plurality of optical signals of which wavelengths are λ1 to λn,and are received by a plurality of receivers Rx1 to Rxn. It should benoted that there is a case where an add/drop multiplexer (ADM) whichadds and extracts an arbitrary wavelength of optical signal is insertedon the optical fiber 85.

It should be noted that, although the above described embodiment 4 is acase where the semiconductor laser element according to the embodiments1, 2-1, and 2-2 or the semiconductor laser module according to theembodiment 3 is used for a pumping light source for use in Ramanamplification, the present invention is not limited to this. It isobvious that, for example, an optical fiber amplifier which is an EDFAcan be realized by using as a pumping light source for EDFA such as 980nm or 1480 nm and by using an optical fiber including erbium which is anamplification medium as an amplification optical fiber.

The semiconductor laser element according to the embodiments of thepresent invention can achieve highly improved efficiency by reducinginternal loss by reducing inter valence band absorption.

Reduction in electric resistance and reduction in thermal resistance arepossible by reducing layer thickness of the p-type cladding layer byreducing internal loss. Also, as compared with a semiconductor laserelement not including an electric-field-distribution-control layer,since the length of a cavity for achieving the same efficiency can belonger, reduction in electric resistance and reduction of thermalresistance are possible. By these effects, driving current and drivingvoltage for obtaining desirable optical output are reduced, low powerconsumption is possible.

Also, since current can be injected to the active layer efficiently byapplying the semiconductor laser element according to the embodiments ofthe present invention to a semiconductor laser element of acurrent-confining structure, a semiconductor laser which is operatedwith low threshold current can be realized. Furthermore, sincerefractive index difference between a current-confining region and anactive layer region can be reduced by interposing anelectric-field-distribution-control layer in the current-confiningregion, a semiconductor laser element which is operable by a singletransverse mode, even if the active layer width is broadened, can berealized. Since electric resistance and thermal resistance can bereduced by this, a high power outputting and low power consumingsemiconductor laser element can be realized in which thermal saturationhardly occurs. Also, since a semiconductor laser element outputtinglaser light which is approximately round beam can be realized by acombination with a buried hetero structure as the current-confiningstructure, a semiconductor laser module which can be coupled with anoptical fiber with high efficiency via an optical component such as alens can be realized.

In addition, the semiconductor laser element according to theembodiments of the present invention does not deterioratecurrent-injecting efficiency to the active layer. Therefore, sinceincrease in voltage, increase in threshold, decrease in efficiency dueto increase in resistance when introducing theelectric-field-distribution-control layer can be restrained, a highlyefficient and low-power-consuming semiconductor laser element can berealized.

In addition, since the semiconductor laser element according to theembodiments of the present invention can realize a low-power-consuminghighly efficient GaInAsP-based semiconductor laser element, higherperformance and lower power consumption for a large-capacitycommunication system can be realized.

In addition, since sum of the layer thicknesses of GaInAsP layersconstituting an electric-field-distribution-control layer is thinnerthan 1 μm according to the semiconductor laser element according to theembodiments of the present invention, a manufacturing process can berealized in which there is fewer crystal defect or surface defect. Bydoing this, a low-power-consuming, high-power-outputting and highlyreliable GaInAsP-based semiconductor laser can be realized at low cost.In addition, in a semiconductor laser element using anelectric-field-distribution-control layer constituted by InP andGaInAsP, ratio including GaInAsP is smaller than a semiconductor laserto which a GaInAsP cladding layer is applied.

Therefore, since thermal resistance can be reduced in a laser moduleassembled with junction up, for example, a ridge-waveguide-type lasermodule or a module using an integrated optical element, awavelength-variable laser which is superior in operation at hightemperature can be realized.

In addition, according to the semiconductor laser element according tothe embodiments of the present invention, since band gap compositionwavelength of the GaInAsP layer constituting theelectric-field-distribution-control layer is equal to or greater than 1μm, a manufacturing process can be realized in which composition controlfor Ga and As is easy. Therefore, a low-power-consuming,high-power-outputting and highly reliable GaInAsP-based semiconductorlaser can be realized at low cost.

In addition, since the semiconductor laser module according to theembodiments of the present invention uses the above describedsemiconductor laser element, there is an effect that a semiconductorlaser module which is operable with low power consumption and high poweroutput can be provided.

In addition, according to the optical fiber amplifier according to theembodiments of the present invention, there is an effect that an opticalfiber amplifier with stable amplification gain and high gain can beprovided by using the above described semiconductor laser element or thesemiconductor laser module.

Also, further effect and variation can be easily derived by thoseskilled in the art.

Therefore, broader aspects of the present invention are not intended tobe limited to the above described embodiments, and various changes arepossible.

As described above, the semiconductor device, the semiconductor lasermodule, and the optical fiber amplifier according to the presentinvention are preferable to be used mainly for optical communication.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. An optical semiconductor device outputting apredetermined wavelength of laser light comprising: a quantum wellactive layer positioned between a p-type cladding layer and an n-typecladding layer in thickness direction; a separate confinementheterostructure layer positioned between the quantum well active layerand the n-type cladding layer; and anelectric-field-distribution-control layer positioned between theseparate confinement heterostructure layer and the n-type cladding layerand configured by at least two semiconductor layers having band gapenergy greater than band gap energy of a barrier layer constituting thequantum well active layer, wherein the optical semiconductor device isapplied to a ridge-stripe type laser.
 2. The optical semiconductordevice according to claim 1, further comprising a current constrictionstructure positioned at both sides of width direction of the quantumwell active layer, wherein the electric-field-distribution-control layeris formed to overlap with the current constriction structure in thethickness direction.
 3. The optical semiconductor device according toclaim 1, wherein the semiconductor layers constituting theelectric-field-distribution-control layer are constituted by a firstsemiconductor layer made from semiconductor material having band gapenergy that is the same as the n-type cladding layer and a secondsemiconductor layer made from semiconductor material having band gapenergy greater than the barrier layer constituting the quantum wellactive layer.
 4. The optical semiconductor device according to claim 3,wherein the first semiconductor layer is made from InP, and the secondsemiconductor layer is made from III-V group compound semiconductorincluding an As atom and a P atom as composition.
 5. The opticalsemiconductor device according to claim 4, wherein the secondsemiconductor layer is made from GaInAsP, and a sum of layer thicknessesof the second semiconductor layers constituting theelectric-field-distribution-control layer is equal to or smaller than 1μm.
 6. The optical semiconductor device according to claim 5, whereinband gap composition wavelength of GaInAsP constituting the secondsemiconductor layer is equal to or greater than 1 μm.